1. Technical Field
The present invention relates to paper transport in image forming apparatuses.
2. Conventional Art
In the paper feed mechanism of an image forming apparatus, locations of drive roller and idler roller serving as paper transport means are fixed, and the direction of transport of paper fed therethrough is constant.
FIG. 7 shows the structure of a paper feed mechanism in a conventional image forming apparatus.
A conventional paper feed mechanism comprises photosensitive body 81 serving as image carrier carrying toner; transfer roller 82, rotating and coming in contact with photosensitive body 81, for causing a toner image formed on this photosensitive body 81 to be electrostatically relocated onto paper 91; and paper transport unit 83 disposed upstream in the transport direction from transfer roller 82 and comprising drive roller (PS roller) 83a and idler roller (PS roller) 83b holding paper 91 in a nip formed therebetween and rotating so as to cause transport of same; this paper transport unit 83 being disposed to the side, on which transfer roller 82 is present, of a plane L more or less tangent to the nip formed between photosensitive body 81 and transfer roller 82 (i.e., disposed below tangent plane L in FIG. 7); and direction R of transport of paper from paper transport unit 83 being set so as to be directed toward the outside circumferential surface 81a of the photosensitive body at a point somewhat to the near side of the nip A formed between photosensitive body 81 and transfer roller 82 (see, e.g., Japanese Patent Application Publication Kokai No. S58-65453 (1983)).
In accordance with such constitution, paper 91, upon being transported from a supply paper storage unit, not shown, is paused as lead edge portion 91a of paper 91 is held in the nip between drive roller 83a and idler roller 83b, paper feed thereafter being carried out with such timing as to cause lead edge 91b of paper 91 to be aligned with the lead edge of the toner image formed on photosensitive body 81; and at this time, in order to cause lead edge 91b of paper 91 to be definitively transported to the transfer unit and made to undergo the transfer operation, transport occurs, as has been stated, such that paper transport direction R is directed toward the outside circumferential surface 81a of the photosensitive body at a point immediately in front of transfer roller 82.
Now it so happens that in recent years there has been increasing diversity in the number of types of paper employed in image forming apparatuses and an increase in the frequency with which card stock and other such heavy-weight paper not employed heretofore has come to be used therein. There are, for example, heavy-weight papers such as card stock which have coated surface(s) and which are used as cover material in bookbinding; such papers are 250 g/m2, which is well over conventional specifications for paper transport (roughly 60 g/m2 to 128 g/m2). Under such circumstances of diverse paper types, if paper transport unit 83 comprising drive roller 83a and idler roller 83b disposed immediately in front of photosensitive body 81 is configured in the foregoing arrangement, movement of the paper at the point of contact will vary depending upon paper rigidity (stiffness) and transport speed of the transported paper. FIG. 8(a) and (b) show the situation at such time.
To wit, as shown in FIG. 8(a), during transport of light-weight paper, stiffness of paper 91 is low, paper 91 being drawn in and transported in smooth fashion as a result of the surface potential of photosensitive body 81 and rotation of photosensitive body 81; but as shown in FIG. 8(b), in the case of heavy-weight paper, stiffness of paper 91 is high, and a phenomenon occurs whereby paper 91 bounces off therefrom before it can be drawn thereinto.
Moreover, in order to cause paper 91, as it leaves paper transport unit 83, to be accurately transported to the nip between photosensitive body 81 and transfer roller 82, a constitution employing paper transport guide plate(s) has been proposed (see, e.g., Japanese Patent Application Publication Kokai No. S58-65453 (1983) and Japanese Patent Application Publication Kokai No. H08-62916 (1996)).
Under such circumstances of diverse paper types, there has been the problem that, if paper transport unit 83 comprising drive roller 83a and idler roller 83b disposed immediately in front of photosensitive body 81 is configured in the foregoing arrangement, a phenomenon has occurred in the case of heavy-weight paper whereby the paper is bounced off therefrom before it can be drawn thereinto, resulting in poor nip entry.
Furthermore, as paper 91 is transported from paper transport unit 83, when the lead edge 91b thereof comes in contact with rotating photosensitive body 81, the lead edge (end region) 91b of the paper hits the photosensitive body surface due to the force of transport and may scratch the photosensitive body surface. In addition, there has also been the problem that as such impact of the photosensitive body surface is repeated, the surface coating layer of the photosensitive body surface is destroyed, causing leakage to occur during the operation in which photosensitive body 81 is charged, resulting in destruction of photosensitive body 81.
Furthermore, with respect to print quality, if the photosensitive body surface layer and/or photosensitive layer is directly scratched by the lead edge of the paper, surface potential at the time of the charging operation will be different there than at other locations (in most cases, resulting in a phenomenon whereby surface potential is decreased thereat), and print quality will suffer due to presence of black lines and/or white lines. Moreover, there has also been the problem that when heavy-weight paper hits photosensitive body 81, this causes drive nonuniformity in the rotation of photosensitive body 81 itself as a result of vibration, and this phenomenon causes occurrence of nonuniformity during writing of image information, resulting in occurrence of band-like nonuniformity in density (banding phenomena) at photosensitive body 81.
In cases such as these, an increase in film thickness at photosensitive body 81 might be considered as one strategy with respect thereto, but this would result in the problem that an increase in film thickness tends to decrease the photosensitivity of photosensitive body 81, and tends to decrease print quality. Furthermore, were film thickness to be increased it would be necessary to apply a voltage greater than that which would otherwise be necessary in order to maintain surface potential at photosensitive body 81; and, particularly in recent years, with the trend toward higher resolutions, when improvement in sensitivity of photosensitive body 81 is being sought, there are many situations where a method involving maintenance of a high surface potential would not be adopted.
Furthermore, the method of installing paper transport guide plate(s) has had the problem that such members are unnecessary when the paper is transported normally, and presence of such paper transport guide plate(s) tends to cause increase in apparatus size. Furthermore, there has also been the problem that, in the event that such transport guide plate(s) are disposed peripherally with respect to photosensitive body 81 or the like, presence of charge at photosensitive body 81 or the like will cause charging of transport guide plate(s), and suspended matter (toner, dust, etc.) flying about within the apparatus could adhere thereto and could soil the paper which is transported therethrough.